Method of producing homogeneous gas mixtures

ABSTRACT

Homogeneous compressed gas mixtures of perfluorinated and/or partially fluorinated hydrocarbons and a gas of lesser density are produced using an optionally mobile mixing station including at least the following components: a gas pre-mixing stage in which separately provided gases are brought together; a static mixer and/or buffer tank connected to the pre-mixing stage; and a compressor connected to the static mixer or buffer tank. If a buffer tank is used, a return line may be provided from the compressor outlet to the buffer tank. Mixtures of perfluorinated and/or partially fluorinated hydrocarbons and/or ethers and inert gases such as noble gases, CO 2  or N 2  and/or SF 6 , are obtained which are suitable as insulating gases for current-carrying underground cables or gas-insulated switches. High flow rates can be processed using the method, and mass flow meters ensure accuracy and reliability.

BACKGROUND OF THE INVENTION

[0001] The invention relates to a method for producing substantiallyhomogeneous, compressed gas mixtures which contain perfluorinated and/orpartially fluorinated hydrocarbons, and to a mixing station, inparticular a mobile one, usable therein.

[0002] In principle, it is possible to convert gases which areseparately present into a homogeneous gas mixture simply, bytransferring the gases into a container and waiting for a sufficientlylong time until a correspondingly homogeneous gas mixture has beenproduced by diffusion. Since however extremely long periods of time arerequired for this, such a method cannot be used industrially. Of course,mixing is also observed if gas streams are introduced into a stationarymixer and/or a common line. However, the mixing is not always sothorough that the mixtures obtained can be regarded as “homogeneous”, inparticular when it is desired to mix together gases having a highdifference in density, for example those which contain perfluorinatedand/or partially fluorinated hydrocarbons. Such gas mixtures can beused, for example, as insulating gas for current-carrying undergroundcables or in gas-insulated circuits. A particular problem in this caseis that the gas mixtures (which are required in very large quantities)expediently need to be produced on the spot. For if it were desired touse gas mixtures prefabricated in a factory, these would have to betransported in gas cylinders under high pressure, in order to keep thetransportation costs as low as possible; however, this is not possible,since then the content of fluorinated hydrocarbons condenses out andcorresponding demixing would occur.

SUMMARY OF THE INVENTION

[0003] It is an object of the present invention to provide a method withwhich homogeneous compressed gas mixtures with fluorinated hydrocarbonsand other gases with a high difference in density can be produced.

[0004] Another object of the present invention is to provide a mixingstation which can be used for this purpose, in particular a mobilemixing station which can be used for this purpose.

[0005] A further object is to provide a mixing station which isprotected from dirt and the effects of the weather.

[0006] These and other objects are achieved in accordance with thepresent invention by providing a method for producing a homogeneouscompressed gas mixture, said method comprising premixing separatelysupplied gases to form a non-homogeneous gas mixture; passing thenon-homogeneous gas mixture into a static mixer or a buffer tank;conveying the gas mixture from the mixer or buffer tank into acompressor; compressing the gas mixture in the compressor; andwithdrawing a substantially homogeneous compressed gas mixture from thecompressor; wherein said gas mixture comprises at least oneperfluorinated or partially fluorinated hydrocarbon or ether.

[0007] Preferably, the gas mixture further comprises at least one gasselected from the group consisting of SF₆ and inert gases, such as noblegases, CO₂ and N₂.

[0008] In accordance with a further aspect, the objects are achieved byproviding a mixing station for carrying out the foregoing method.

[0009] According to the invention, compounds selected from the groupconsisting of perfluorinated and/or partially fluorinated hydrocarbonsare used as fluorinated hydrocarbons for the production of substantiallyhomogeneous compressed gas mixtures from gases which are separatelypresent.

[0010] Fluorinated compounds within the context of the invention are tobe understood to mean perfluorinated and/or partially fluorinatedhydrocarbons which can be pressure-liquefied, in particular thosecompounds which develop a vapour pressure of <30 bar (abs.) at 50° C.Suitable representatives of these classes of substances include, forexample, R218 (C₃F₈), R125 (CHF₂CF₃), R227ea (CF₃CHFCF₃), R134a(CH₂FCF₃), R143a (CH₃CF₃), R404 (R125/R143a/R134a), R23 (CHF₃), R14(CF₄), R116 (CF₃CF₃) or E125 (CF₃OCHF₂). Suitable mixture constituentsinclude, for example, SF₆, inert gases, e.g. noble gases, CO₂ or N₂.

[0011] The method provides for the gases which are supplied separatelyto be premixed, forming a non-homogeneous gas mixture, thenon-homogeneous gas mixture to be passed into a static mixer and/orbuffer tank, the gas mixture to be passed from the buffer tank or thestatic mixer into a compressor, and a substantially homogeneouscompressed gas mixture to be delivered from the compressor, wherein, ifa buffer tank is provided, a portion of the substantially homogeneouscompressed gas mixture delivered from the compressor is returned intothe buffer tank via a return line.

[0012] The method according to the invention makes it possible toproduce homogeneously mixed gas mixtures at the point of use. It istherefore no longer necessary to supply gas mixtures homogeneously mixedex works. Another advantage is that high flow rates (for example above200 standard m³ per hour) can be processed. In this case, the degree ofmixing is independent of the cross-sections of the lines used. Metereddelivery of the final homogeneous gas mixture is possible.

[0013] If a static mixer and a buffer tank are provided, it isadvantageous to pass the gas first through the static mixer and thenthrough the buffer tank.

[0014] In accordance with one preferred embodiment, the method isperformed using a buffer tank, and a control valve is installed in thereturn line. The return of a portion of the gas mixture is adjusted tothe desired value with this control valve. This embodiment has theadvantage that the compressor can be operated under gas ballast, and inaddition the thorough mixing is improved still further. The controlvalve may, for example, be adjusted such that a predetermined proportionof the volume of the compressed gas delivered from the compressor isreturned.

[0015] Advantageously, a safety means is provided which registers thefact that the filling limit has been reached in the electric cable whichis to be filled or the gas cylinder which is to be filled, and switchesoff the compressor. This may, for example, be a pressure-relief valve,which from a predetermined pressure onwards opens and advantageouslyturns off the compressor. The pressure-relief line may be connected tothe buffer tank. In this manner, the gas released through the pressurerelief valve remains in circulation.

[0016] The compressor is adjusted such that it supplies a gas mixturehaving the desired pressure. The aforementioned gas mixtures areadvantageously delivered at a pressure of 1 to 13 bar absolute. Inparticular, the pressure is in the range from 4 to 9 bar absolute.

[0017] Compressors which operate without oil, in particular diaphragmcompressors, but also piston compressors, are advantageously used.

[0018] The quantities of gas which are supplied to form gas mixtures ofgiven composition are preferably controlled via mass flow meters. Thisis advantageous precisely for gases with a high difference in density;the quantities of gas may be controlled accurately despite variabletemperatures (influence of the time of day or year).

[0019] Preferably the method according to the invention can be used forthe production of mixtures from gases in which at least one gasconstituent is pressure-liquefied. Preferably CHF₂CF₃, CF₃CF₃, C₃F₈,CH₂FCF₃, CH₃CF₃, CF₃CHFCF₃ or SF₆ are used as pressure-liquefiablegases.

[0020] The method is particularly well suited for producing homogeneousgas mixtures which contain or consist of CF₄, CHF₂CF₃, CF₃CF₃, C₃F₈ andN₂ or SF₆. Such gas mixtures are used for example as an insulating gasfor current-carrying underground cables or in gas-insulated circuits.

[0021] In gas-insulated switchgear or in gas-insulated circuits, the gasmixtures according to the invention are preferably used in areas inwhich no arcs are produced.

[0022] The method according to the invention is particularly well suitedfor the production of substantially homogeneous compressed gas mixtures.Preferably the desired content of the gas mixture in a sample takendeviates by at most ±0.7% by volume from the value which occurs withideal thorough mixing (i.e., ideal homogeneity). Optionally theproportion of the already-mixed gas returned to the buffer tank via thereturn line is increased. The analysis may be by e.g. gaschromatography.

[0023] The composition of the gas mixtures may vary within a wide range.For example, homogeneous gas mixtures can be manufactured which contain5 to 95% by volume, preferably 40 to 90% by volume, in particular 50 to95% by volume, of perfluorinated and/or partially fluorinatedhydrocarbons. The remainder to make up to 100 % by volume is accordinglythe other constituent such as SF₆ or an inert gas, preferably N₂.

[0024] Mixtures which contain only partially fluorinated andperfluorinated hydrocarbons, such as mixtures of 20% by volume R218 and80% by volume R125, can likewise be manufactured and are suitable foruse as an insulating gas.

[0025] One preferred embodiment of the method according to the inventionprovides for a gas mixture containing or consisting of perfluorinatedand/or partially fluorinated hydrocarbons and N₂ to be produced and forthis to be introduced as an insulating gas into current-carryingunderground cables or gas-insulated switchgear.

[0026] The invention also relates to a mixing station which can be usedto perform the method according to the invention to produce gas mixtureswith perfluorinated hydrocarbons and substantially lighter gases. Thismixing station comprises the following components: at least two feedlines for feeding the gases to be mixed; a gas line for jointly passingon the premixed gases; a stationary mixer and/or a buffer tank intowhich the gas line for jointly passing on the premixed gases opens; agas line which is connected to the buffer tank or stationary mixer and acompressor, through which gas mixture is passed from the buffer tank orthe stationary mixer into the compressor; a compressor in which the gasmixture carried off from the buffer tank or stationary mixer iscompressed and homogenised; a removal line for carrying the homogeneouscompressed gas mixture out of the compressor. If a buffer tank ispresent, the mixing apparatus may also comprise a return line connectedbetween the removal line from the compressor and the buffer tank, and acontrol valve in the return line. The feed lines for the gases to bemixed may be connected via a T-type connector to the gas line forjointly passing on the gases. One preferred embodiment of the mixingstation has a buffer tank and a return line with control valve.

BRIEF DESCRIPTION OF THE DRAWINGS

[0027] The invention will be described in further detail hereinafterwith reference to illustrative preferred embodiments depicted in theaccompanying drawing figures, in which:

[0028]FIG. 1 is a schematic illustration of a simple mixing station forcarrying out the method of the invention, and

[0029]FIG. 2 is a schematic diagram illustrating how the method of theinvention is carried out.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0030]FIG. 1 shows a simple mixing station which comprises two feedlines (1, 2); two valves (3, 4) for regulating the gas flow rate; a gasline (5) for passing on the premixed gases; a buffer tank (6); acompressor (7); a gas line (8) between the buffer tank (6) and thecompressor (7); a removal line (9); a return line (10) between thebuffer tank and the compressor; a control valve (11) in the return line;and a valve (12) for regulating the quantity of the homogeneous gasmixture removed.

[0031] The mixing station may comprise further useful components such asone or more manometers, pressure reducers, flow meters, pressure-reliefvalves, automatic shut-off means for the compressor, removal points fortaking samples or a removal point for the homogeneous gas mixture.Particularly advantageously, the device comprises mass flow meters inorder to regulate the quantities of gas. A collecting device of thistype delivers accurate results independently of the temperature (time ofday, time of year) at which it is operated—despite the high differencesin gas density.

[0032] The mixing station may furthermore comprise: at least one holderfor holding gas cylinders for one or more of the unmixed gases; aconnection for connecting a gas cylinder for receiving the homogeneouscompressed gas mixture; and/or at least one holder for such a gascylinder.

[0033] It may furthermore comprise means for protecting from externalinfluences. For example fittings with a tarpaulin may be provided, whichkeep dirt and the effects of the weather away from it.

[0034] The mixing station may be mobile. It then comprises the mixingstation described above and an undercarriage on which the mixing stationis mounted. For example, the undercarriage may be a truck or a trailer.This has the advantage that the mixing station can be moved along withthe laying of underground cables to be insulated.

[0035] The method according to the invention will be explained ingreater detail with reference to FIG. 2. The fluorinated hydrocarbonsand nitrogen are introduced into a gas mixer (G) from the tank (ST) orthe nitrogen tank (NT) via evaporators (V), manometers (M) and pressurereducers (D). The pressure between the manometer and the pressurereducer is 9 to 15 bar. In the gas mixer, the two gases are introducedinto a common line (5) via mass flow meters and butterfly valves. Thedifferential pressure between (M) and the static mixer (F) is at least 3bar. The premixed gas is introduced into the buffer tank (6) via thestatic mixer (F) and is introduced from the buffer tank via line (8)into the compressor (7). A portion of the gas removed from thecompressor via line (9) is returned into the buffer tank via line (10)and the control valve (11). The pressure in the line (9) may range up to13 bar (i.e. 14 bar absolute). Gas samples can be taken for analysis viathe sampling points (13, 13′) and (13″). The flow rate in line (9) isfrom 5 to 250 standard m³ per hour. Homogeneous gas mixture isintroduced into a gas cylinder, not shown here, via line (9). Thecontrol valve (11) is set such that the desired degree of thoroughmixing is achieved—the greater the volume percent which is recycled tothe buffer tank, the more ideal the thorough mixing, but of course alsothe lower the quantity of compressed gas mixture delivered. Thecompressed gas is delivered via the shut-off valve (14) to the articleto be filled (e.g. an electric cable, switch housing or a gas cylinder).

[0036] The following examples are intended to illustrate the inventionin further detail without limiting its scope.

EXAMPLES Example 1

[0037] The flow rates from the tank or the nitrogen tank were adjustedsuch that the volume ratio of C₃F₈:N₂ was exactly 40:60. Samples whichwere taken directly after the gas buffer tank and from the gas cylindereach had a content of 40% by volume C₃F₈ and 60% by volume N₂, and provethat optimum thorough mixing took place.

Example 2

[0038] The test was repeated, with a volume ratio of 50:50 being set. Asample taken after the buffer tank contained in each case 50% by volumeof the mixture constituents.

Examples 3-7

[0039] Ex. Constituent 1 Constituent 2 Mixture ratio Result 3 R125 SF₆80:20 Homogeneous gas mixture 4 R404A SF₆ 80:20 Homogeneous gas mixture5 R404A N₂ 50:80 Homogeneous gas mixture 6 E125 SF₆ 70:30 Homogeneousgas mixture 7 R218 N₂ 20:80 Homogeneous gas mixture 8 R218 R125 20:80Homogeneous gas mixture 9 R227 R125 50:50 Homogeneous gas mixture

[0040] In all the examples, it was possible to stay within the deviationof ±0.7% by me of the target mixture.

[0041] The foregoing description and examples have been set forth merelyto illustrate the invention and are not intended to be limiting. Sincemodifications of the described embodiments incorporating the spirit andsubstance of the invention may occur to persons skilled in the art, theinvention should be construed broadly to include all variations withinthe scope of the appended claims and equivalents thereof.

What is claimed is:
 1. A method for producing a homogeneous compressedgas mixture, said method comprising premixing separately supplied gasesto form a non-homogeneous gas mixture; passing the non-homogeneous gasmixture into a static mixer or a buffer tank; conveying the gas mixturefrom the mixer or buffer tank into a compressor; compressing the gasmixture in the compressor; and withdrawing a substantially homogeneouscompressed gas mixture from the compressor; wherein said gas mixturecomprises at least one perfluorinated or partially fluorinatedhydrocarbon or ether.
 2. A method according to claim 1, wherein said gasmixture further comprises at least one gas selected from the groupconsisting of SF₆ and inert gases.
 3. A method according to claim 2,wherein said mixture comprises an inert gas selected from the groupconsisting of noble gases, CO₂ and N₂.
 4. A method according to claim 1,wherein said mixture comprises at least one perfluorinated or partiallyfluorinated hydrocarbon or ether and SF₆.
 5. A method according to claim1, wherein said mixture comprises at least one perfluorinated orpartially fluorinated hydrocarbon and N₂.
 6. A method according to claim5, wherein said mixture consists of at least one perfluorinated orpartially fluorinated hydrocarbon and N₂.
 7. A method according to claim1, wherein said mixture comprises at least one perfluorinated orpartially fluorinated hydrocarbon selected from the group consisting ofC₃F₈, CHF₂CF₃, CF₃CHFCF₃, CH₂FCF₃, CH₃CF₃, CHF₃, CF₄, CF₃CF₃, CF₃OCHF₂.8. A method according to claim 1, wherein the compressed gas mixturewithdrawn from the compressor has a pressure of up to 13 bar.
 9. Amethod according to claim 1, wherein the non-homogeneous gas mixture ispassed into a buffer tank and conveyed from the buffer tank to thecompressor; further comprising returning a portion of the compressed gasmixture withdrawn from the compressor through a return line to thebuffer tank, and wherein a control valve is installed in the return linefor adjusting the return of compressed gas mixture to a desired volumepercentage of the compressed gas withdrawn from the compressor.
 10. Amethod according to claim 1, wherein a homogeneous compressed gasmixture is produced having a composition which deviates by at most ±0.7volume-% from ideal homogeneity.
 11. A method according to claim 1,wherein gas streams which are to be mixed are regulated using mass flowmeters.
 12. A method according to claim 11, wherein said method iscarried out in a mobile mixing apparatus.
 13. A method according toclaim 1, further comprising introducing the substantially homogeneouscompressed gas mixture as an insulating gas into a current-carryingunderground cable or a gas-insulated switch.